libcurl-thread

NAME

Multi-threading with libcurl

libcurl is thread safe but has no internal thread synchronization. You may have
to provide your own locking should you meet any of the thread safety exceptions
below.

Handles. You must never share the same handle in multiple threads.
You can pass the handles around among threads, but you must never use a single
handle from more than one thread at any given time.

Shared objects. You can share certain data between multiple handles by
using the share interface but you must provide your own locking and set
curl_share_setopt(3) CURLSHOPT_LOCKFUNC and CURLSHOPT_UNLOCKFUNC.

TLS

If you are accessing HTTPS or FTPS URLs in a multi-threaded manner, you are
then of course using the underlying SSL library multi-threaded and those libs
might have their own requirements on this issue. You may need to provide one
or two functions to allow it to function properly:

OpenSSL

OpenSSL 1.1.0 "can be safely used in multi-threaded applications provided that
support for the underlying OS threading API is built-in."

Other areas of caution

Signals

Signals are used for timing out name resolves (during DNS lookup) - when built
without using either the c-ares or threaded resolver backends. When using
multiple threads you should set the CURLOPT_NOSIGNAL(3) option to 1L for
all handles. Everything will or might work fine except that timeouts are not
honored during the DNS lookup - which you can work around by building libcurl
with c-ares or threaded-resolver support. c-ares is a library that provides
asynchronous name resolves. On some platforms, libcurl simply will not
function properly multi-threaded unless the CURLOPT_NOSIGNAL(3) option is
set.

Name resolving

gethostby* functions and other system calls. These functions, provided
by your operating system, must be thread safe. It is very important that
libcurl can find and use thread safe versions of these and other system calls,
as otherwise it can't function fully thread safe. Some operating systems are
known to have faulty thread implementations. We have previously received
problem reports on *BSD (at least in the past, they may be working fine these
days). Some operating systems that are known to have solid and working thread
support are Linux, Solaris and Windows.

curl_global_* functions

These functions are not thread safe. If you are using libcurl with multiple
threads it is especially important that before use you call
curl_global_init(3) or curl_global_init_mem(3) to explicitly
initialize the library and its dependents, rather than rely on the "lazy"
fail-safe initialization that takes place the first time
curl_easy_init(3) is called. For an in-depth explanation refer to
libcurl(3) section GLOBAL CONSTANTS.

Memory functions

These functions, provided either by your operating system or your own
replacements, must be thread safe. You can use curl_global_init_mem(3)
to set your own replacement memory functions.